DoL Retainers?

[Niwashi]

Why would Botany be useless now? Somehow I have it stuck in my head that Botany gathers mats to support CUL & ALC, the 2 crafting jobs I want to eventually acquire

I'm not sure why Botany would be considered worthless. (There may be some endgame reason I haven't gotten to yet, though.) Yes, it does gather most of the crafting components for CUL and ALC.

Is it true that once equipped, gear I assign to the retainer can not be used on my main (I'd assume so, just checking to make sure)?

No. Your character and retainer can't both wear the same item at the same time, and you cannot change your retainer's gear while he/she is in the midst of a venture, but between ventures you can swap something back and forth between them. I'll occasionally switch my best (Ironworks) gear which I usually use myself as Bard onto my archer retainer so he can have the higher item level long enough for a quick venture or two on days when I'm focused on crafting or leveling my other classes.

One other note about retainer gear:
For DoL retainers, the only stat that affects their results is Gathering. There's no need to equip them with the other DoL gear that improves Perception or GP.
For DoW/DoM retainers, the only stat that affects their results is Item Level. Gear has to be wearable by that class, but it doesn't matter whether its stats would normally be suited to the class or not.

AND are the ventures I assign to her going to be specific to acquiring ore, plants, or fish?

There are three types of ventures you can assign:

A Hunting, Mining, Botany, or Fishing venture takes 1 hour (or maybe less if it's a high level retainer going on a low level venture), costs 1 venture token, and returns several of an item of your choice (with the list of choices being dependent on your retainer's class and level). For this type of venture, you have to have already gathered the item at least once yourself (though that's not true of the other two types of ventures).

A Quick Venture takes 1 hour, costs 2 venture tokens, and returns a random item. It may or may not be related to your retainer's class. It might, if you're lucky, be one of the rare items that are exclusively available through ventures (or if you're not so lucky, it might be nearly worthless junk). The amount of experience you get is variable, depending on the quality of the item brought back. Your retainer has to level up a little (I think it's level 10) before you can do this type of venture.

A Field, Highland, Woodland, or Waterside Exploration Venture (which are all essentially the same thing, just named according to your retainer's class) takes 12 18 hours, costs 2 venture tokens, and returns a random item (or a few of a random item) and one or more Allagan Bronze or Silver Pieces. It gives more Experience than a Hunting, Mining, Botany, or Fishing venture, and more than most Quick Ventures. (Though it's possible that some of the best Quick Venture results exceed it's XP. I'm not entirely sure on that part.)